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Re: Could synchronous streaming replication really degrade the performance of the primary?

From: Claudio Freire <klaussfreire(at)gmail(dot)com>
To: Robert Klemme <shortcutter(at)googlemail(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Could synchronous streaming replication really degrade the performance of the primary?
Date: 2012-05-09 15:45:50
Message-ID: CAGTBQpYXUeWm1O+rc4NCZxeFB8Oxi8LJuSme-As9b4m6iU0Nbg@mail.gmail.com (view raw or flat)
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Lists: pgsql-performance
On Wed, May 9, 2012 at 12:41 PM, Robert Klemme
<shortcutter(at)googlemail(dot)com> wrote:
> I am not sure whether the replicant can be triggered to commit to disk
> before the commit to disk on the master has succeeded; if that was the
> case there would be true serialization => 50%.
>
> This sounds like it could actually be the case (note the "after it commits"):
> "When synchronous replication is requested the transaction will wait
> after it commits until it receives confirmation that the transfer has
> been successful."
> http://wiki.postgresql.org/wiki/Synchronous_replication

That should only happen for very short transactions.
IIRC, WAL records can be sent to the slaves before the transaction in
the master commits, so bigger transactions would see higher
parallelism.

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